Design and testing of a compact layered isolator for high-frequency helicopter gearbox isolation

Meshing gear pairs in rotorcraft transmissions are a significant source of vibration and cabin noise. This tonal, high-frequency gearbox noise (500 Hz - 2000 Hz) is primarily transmitted to the fuselage through rigid connections, which do not appreciably attenuate vibratory energy. Because periodically-layered elastomer and metal isolators exhibit transmissibility "stop bands," or frequency ranges in which there is very low transmissibility, they may provide an elegant passive vibration control solution. Analytical investigations suggest that the addition of fluidic, inertial amplifiers between layers is particularly effective in significantly lowering the beginning stop band frequencies without adding excessive mass. Also, the tuned fluid masses together with the stiffness of the elastomer layers cause internal, tuned vibration absorber effects. A preliminary, multi-layered fluidic isolator was fabricated and tested to experimentally validate the utility of fluidic elements as high-frequency inertial amplifiers. This initial specimen, however, is relatively heavy and generally unfeasible for incorporation into gearbox suspensions. Two configurations of a new, lightweight, compact specimen were therefore designed subject to estimated gearbox isolation design constraints. Experimental and analytical transmissibilities of both the preliminary and compact specimens are compared. The compact specimen was capable of reducing transmitted vibrations by factor of 100 within the stop band. Also, the effect of precompression on elastomer stiffness properties was experimentally investigated and axial quasi-static stiffness tests were conducted to compare analytical stiffness predictions to experimental results.